Push-button signal receiving circuit and a method of detecting a push-button signal

ABSTRACT

A push-button signal receiving circuit and a push-button signal detection method are provided which can enhance frequency detection accuracy and yet can reduce erroneous determination as to the valid length of PB signal and thus erroneous operation. Valid signal determining portion determines the validity of a received PB signal in accordance with the results of determination as to the coincidence of continuance times of identification signals extracted by first and second frequency detecting portion for performing frequency detection over a small number of periods, and the coincidence of the frequencies detected by the first and second frequency detecting portion with those detected by third and fourth frequency detecting portion over a large number of periods.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit for receiving a push-buttonsignal and identifying a dialed number based on the received signal, andto a method of detecting such a push-button signal.

2. Description of the Related Art

In analog telephone communications, push-button signal (hereinafterabbreviated as PB signal) is used as a selection signal whereby atelephone terminal specifies a target of connection. In suchcommunications using the PB signal, one of low-frequency identificationsignals with frequencies of 697 Hz, 770 Hz, 852 Hz and 941 Hz iscombined with one of high-frequency identification signals withfrequencies of 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz so that based on atotal of 16 different signals, a dialed number can be identified. Alow-frequency signal and a high-frequency signal, which are used incombination, are continuously sent out for a fixed time or longer inresponse to a single dialing operation. Also, between dealingoperations, there is provided a time period which is equal to or longerthan a second fixed time called minimum cause and in which no signal ispresent.

A PB signal receiving circuit receives the combination of the low- andhigh-frequency signals, and judges that the input signal is a validselection signal if the signal has been continuously received over thefirst fixed time. Also, if reception of the signal discontinues for thesecond fixed time or longer, the PB signal receiving circuit regardssuch a signal discontinuance time as the minimum pause, and judges asignal with the same frequency received thereafter to be a differentselection signal.

FIG. 6 exemplifies a schematic configuration of a conventional PB signalreceiving circuit generally used.

A PB signal receiving circuit 30 shown in FIG. 6 comprises a filter 31for removing a dial-tone signal from the input signal, filters 32 and 33for separating the input signal into low- and high-frequency signals,respectively, frequency detection circuits 34 and 35 for the low- andhigh-frequency signals, respectively, a signal determination circuit 36for identifying a dialed number based on the detected frequencies, acontrol circuit 37 for determining valid length as the identificationsignal, and an output circuit 38 for outputting number data with a validlength.

The filter 31 removes, from the input signal, the dial-tone signal whichis an audible signal with a frequency lower than the low-frequencysignals and which prompts the user of a telephone terminal to send aselection signal. The filters 32 and 33 selectively pass only low- andhigh-frequency bands, respectively, of the signal output from he filter31. The frequency detection circuits 34 and 35 detect the frequencies ofthe respective input signals. If both of the frequency detectioncircuits 34 and 35 detect valid low and high frequencies, respectively,the signal determination circuit 36 recognizes detection of low- andhigh-frequency identification signals and outputs a detection signal tothe control circuit 37. Also, the signal determination circuitidentifies a dialed number based on the combination of theidentification signals and outputs number data indicative of theidentified number to the output circuit 38. The control circuit 37monitors the continuance time and discontinuance time of the inputdetection signal to determine valid length as the identification signal,and outputs a validity signal EN indicating the length. The outputcircuit 38 outputs number data having a length based on the validitysignal EN from the control circuit 37, as 4-bit data D31, D32, D33 andD34, for example.

Referring now to FIG. 7 which is a timing chart showing signalsappearing in various parts of the above PB signal receiving circuit 3C,the operation of the receiving circuit 30 will be described.

A input signal S41 input to the PB signal receiving circuit 30 includesthe dial tone, low-frequency signal, high-frequency signal, etc. Thefilter 31 outputs a signal S42 from which the dial tone has beenremoved, and the filters 32 and 33 output signals S43 and S44 containingonly components of the low- and high-frequency ranges, respectively. Thefrequency detection circuits 34 and 35 output identification signals S45and S46, which correspond to their respective assigned low and highfrequencies, to the signal determination circuit 36. The signaldetermination circuit 36 outputs a detection signal S47 corresponding tothe PB signal, which is derived as a logical product of theidentification signals S45 and S46, to the control circuit 37.

The control circuit 37 monitors the continuance time and discontinuancetime of the input of the detection signal S47, and outputs the validitysignal EN indicating a valid length as the selection signal.Specifically, the control circuit 37 starts to count the inputcontinuance time at timing T21 at which The detection signal S47 turnsto the H level, for example. If the detection signal S47 remains at theH level up to timing T22 over a preset continuance criterion time Ton,the control circuit judges that the detection signal S47 is a validselection signal, and turns the validity signal EN to the H level. Attiming T23 at which the detection signal S47 turns to the L level, thecontrol circuit starts to count the input discontinuance time. However,since the detection signal S47 again turns to the H level at timing T24before a preset discontinuance criterion time Toff elapses, the controlcircuit does not judge that the selection signal has discontinued, andholds the validity signal EN at the H level. The control circuit againstarts to count the discontinuance time at timing T25 at which thedetection signal S47 turns to the L Level, and at timing T26 after alapse of the discontinuance criterion time Toff for which the detectionsignal remained at the L level, the control circuit judges that theselection signal has discontinued, and turns the validity signal EN tothe L level. The output circuit 38 outputs the number data D31 to D34having a valid data length based on the rise and fall timings of thevalidity signal EN.

FIG. 8 shows frequency detection characteristics of the PB signalreceiving circuit 30.

As shown in FIG. 8, in the PB signal receiving circuit 30, allowablefrequencies fah and fal are set respectively as upper and lower limitsof frequency for allowing signal reception, with respect to a nominalfrequency fo of each identification signal, and a signal with afrequency falling within the range between the allowable frequencies fahand fal is received without fail. Also, forbidden frequencies fph andfpl are set so as to be separated from the allowable frequency range,and a signal with a frequency higher than the forbidden frequency fphand a signal with a frequency lower than the forbidden frequency fpl arenot accepted. A region between the allowable frequency fah and theforbidden frequency fph and a region between the allowable frequency faland the forbidden frequency fpl each constitute an uncertainty region inwhich whether signal is received or not is uncertain. In order toprevent erroneous operation, therefore, the range between the allowablefrequencies fah and fal should desirably be narrowed, and also thevalues of the forbidden frequencies fph and fpl should desirably be asclose to the respective allowable frequencies fah and fal as possible.

Provided a frequency deviation in which reception is allowed is da and afrequency deviation in which reception is forbidden is dp (da<dp), thenominal frequency fo, the allowable frequencies fah and fal, and theforbidden frequencies fph and fpl are in the relationships indicated bythe following equations (1), (2), (3) and (4):fah=(1+da)×fo  (1)fal=(1−da)×fo  (2)fph=(1+dp)×fo  (3)fpl=(1−dp)×fo  (4)

Meanwhile, a method of detecting frequency in the frequency detectioncircuits 34 and 35 includes a method of extracting a signal with aspecified frequency by using a filter and a method of measuring period.Of these methods, the period measuring method is more often used becauseof simplicity of circuit configuration and hence higher economicalefficiency.

To measure the period, an interval between time points at which theinput signal level crosses a certain threshold is measured, for example.In cases where the input signal includes noise whose frequency fallswithin the passband of the filter 32 or 33, however, such noise can varythe threshold crossing timing, thus causing jitter. FIG. 9 illustratesnoise-induced period variations during the period measurement.

In FIG. 9, the threshold for frequency detection is set at 0 V. Also, inthe figure, a signal S51 shows an example of input signal waveformincluding no noise, and signals S52 and S53 individually show the inputsignal on which noise is superimposed in a range such that the thresholdcrossing timing is varied by +Δt at the maximum. Provided the peak levelof the signal S51 is S, the noise level with respect to the signal S51is N, and the period of the signal S51 is T, thensin(2π×(Δt/T))=N/S  (5)Δt=T×sin⁻¹(N/S)/2π  (6)

As seen from equation (6) above, the variation Δt in the thresholdcrossing point is dependent on the ratio of the noise level N to peaklevel S of the signal S51. Also, because of inclusion of noise, adetected period varies in the range from a minimum value Tmin (=T−2Δt)to a maximum value Tmax (=T+2Δt), as shown in FIG. 9. Jitter J caused inthis case is indicated by equation (7) below.J=2Δt/T=sin⁻¹(N/S)/π  (7)

FIG. 10 is a graph showing the relationship between the amplitude ratioN/S of noise to signal and jitter J, based on equation (7), and FIG. 11illustrates the influence of jitter upon the frequency detectioncharacteristics.

As shown in FIG. 10, the amount of jitter J increases with increase inthe amplitude ratio N/S of noise to signal. Also, as shown in FIG. 11,in the case of allowing the reception of noise in a range in whichjitter J is caused, then it is necessary that the absolute value of theallowable frequency deviation for the frequency detection should beda+J, and that the absolute value of the forbidden frequency deviationshould be dp−J. Thus, in the case where the frequency range in whichnoise is admitted at the time of frequency detection is broadened, theforbidden frequency deviation also should be enlarged. This, however,lowers the frequency detection accuracy often causes an error indetermining the presence/absence of input signal.

Usually, ambient noise picked up by the microphone of a telephoneterminal or noise caused by crosstalk of lines, etc. is superimposed onthe signal input to the PB signal receiving circuit 30, and it istherefore desirable that the identification signal be detected whileallowing inclusion of a certain degree of noise. However, in theconventional P3 signal receiving circuit 30, broadening the range ofadmitting noise entails an increase in the rate of occurrence of errors,as mentioned above. In order to enhance the accuracy in the frequencydetection of the input signal while at the same time allowing inclusionof a certain degree of noise, a method may be adopted in which thefrequency (period) detection cycle is set to multiple periods, not oneperiod, of the input signal. If the detection cycle is set to n periodsof the input signal, for example, the influence of jitter J on thefrequency detection can be reduced to 1/n.

However, where frequency is detected over multiple periods of the inputsignal, a longer time is required before the detected frequency isestablished than in the case of the detection over a smaller number ofperiods, such as one period, so that error in determining thecontinuance and discontinuance of the input signal expands. FIG. 12shows examples of frequency detection over multiple periods of the inputsignal, wherein (A) shows the input signal, (B) shows the frequencydetermination according to a first example of detection, (C) shows theoutput of the identification signal according to the first example ofdetection, (D) shows the frequency determination according to a secondexample of detection, and (E) shows the output of the identificationsignal according to the second example of detection.

FIG. 12 illustrates in time sequence how a low-frequency identificationsignal with a frequency of 697 Hz, for example, is detected, wherein thenumber n of periods of the input signal corresponding to one detectioncycle is 20 (n=20). Thus, one detection cycle in which the detectedfrequency is established is 28.69 msec. Also, a maximum time for thedetection over 20 periods of the input signal is set to 31.56 msec,which is 110% of one detection cycle. If 20 periods of the input signalexceed 31.56 msec, it is judged that the input signal is not valid asthe 697-Hz identification signal, and the next period measurement of theinput signal is started. Parts (C) and (E) of FIG. 12 illustrate thestates of extraction of the identification signal S45 by the frequencydetection circuit 34 appearing in FIG. 6.

As shown in (A) of FIG. 12, the input signal is actually input to thefrequency detection circuit 34 from timing T32 to timing T40. In thefirst example of detection shown in (B) of FIG. 12, the period detectionis started at timing T33 immediately after timing T32 of the inputsignal, and the detection time for 20 periods is counted. Then, attiming T35 after a lapse of 28.69 msec from the start of detection,reception of the 697-Hz Identification signal is detected, and theidentification signal is extracted as shown in (C) of FIG. 12. Thecounting of the period detection time is again started thereafter forthe next detection.

In the first example of detection, at timing T40 immediately after thedetection is newly started at timing T39, the input signal discontinues.In this detection cycle, therefore, the input is judged invalid attiming T42 after a lapse of 31.56 ms, and the output of theidentification signal is stopped, as shown in (C) of FIG. 12.

On the other hand, in the second example of detection shown in (D) ofFIG. 12, the period detection is newly started at timing T31 and theperiod detection time is counted. However, since the input of the signalstarts at timing T32 immediately after the start of the perioddetection, 20 periods of the input signal fail to be detected within31.56 ms. Accordingly, the input signal is judged invalid at timing T34,and the counting of the period detection time is again started. As aresult of the subsequent period detection terminating at timing T36, theidentification signal is extracted as shown in (E) of FIG. 12.

Also, the detection is newly started at timing T38, and the input signaldiscontinues at timing T40 immediately before the counting of 20 periodsis finished. In this detection cycle, therefore, the input is judgedinvalid at timing T41 after a lapse of 31.56 ms, and the output of theidentification signal is stopped, as shown in (E) of FIG. 12.

As seen from the above, in the case where the detection cycle starttiming is immediately after the reception start timing of the inputsignal, as in the first example of detection, the output start timing ofthe identification signal is earlier by a maximum of about one detectioncycle, that is, about 28.69 msec, than in the case where the detectionstart timing is immediately before the reception start timing of theinput signal, as in the second example of detection. Also, where thereception of the input signal ends immediately after the completion ofcounting for 20 periods, as in the first example of detection, theoutput stop timing of the identification signal is later by a maximum ofabout one detection cycle than in the case where the reception of theinput signal ends immediately before the completion of counting for 20periods, as in the second example of detection. Accordingly, the outputtiming of the identification signal is subject to an error of about twodetection cycles at the maximum. Thus, in the case of the perioddetection over multiple periods of the input signal, the error in theoutput timing of the identification signal expands because of longdetection cycle.

FIG. 13 illustrates exemplary cases where a short break occurs duringthe frequency detection over multiple periods of the input signal,wherein (A) shows the input signal, (B) shows the frequencydetermination according to a first example of detection, (C) shows theoutput of the identification signal according to the first example ofdetection, (D) shows the frequency determination according to a secondexample of detection, and (E) shows the output of the identificationsignal according to the second example of detection.

In FIG. 13, the identification signal with a frequency of 697 Hz isdetected, n is set to 20 (n=20), so that one detection cycle is 28.69msec, and a maximum allowable time for the detection of 20 periods isset to 31.56 msec, which is 110% of one detection cycle, as in FIG. 12.

As shown in (A) of FIG. 13, the signal actually input to the frequencydetection circuit 34 undergoes a short break from timing T53 to timingT55. In the first example of detection shown in (B) of FIG. 13, thedetection cycle starts anew at timing T52, and after the counting ofsignal continuance time is started, the input signal discontinues fromtiming T53 to timing T55. Since the period detection of the input signalis delayed because of the discontinuance time, 20 periods of the inputsignal fail to be detected within 31.56 ms; accordingly, the inputsignal is judged invalid at timing T56 and the output of theidentification signal is stopped, as shown in (C) of FIG. 13.Subsequently, the counting of the period detection time is againstarted, and the output of the identification signal is started again attiming T58 as a result of the frequency detection.

On the other hand, in the second example of detection shown in (D) ofFIG. 13, the period detection is newly started at timing T51, and theinput signal discontinues at timing T53 before the input signal isreceived over 20 periods. At timing T54 within the discontinuance time,the maximum time 31.56 ms as counted from the start of the perioddetection elapses, so that the input is judged invalid, and the outputof the identification signal is stopped, as shown in (E) of FIG. 13.Further, while the input signal is discontinued, the period detection isagain started; therefore, also in the subsequent detection cycle, 20periods of the input signal fail to be detected within 31.56 ms. Thus,the input signal is again judged invalid at timing T57, so that noidentification signal is output. Subsequently, the frequency detectionis newly started, and the output of the identification signal s startedagain at timing T59.

As seen from the above, in the case where the input signal is judgedinvalid at timing during the short break of the input signal, as in thesecond example of detection, the output stop time of the identificationsignal is prolonged for a maximum of about one detection cycle, comparedwith the first example of detection wherein the signal determinationdoes not take place during the short break. Consequently, error in theoutput stop time of the identification signal expands with increase inthe detection cycle.

As shown in the examples of FIGS. 12 and 13, in the case of detectingthe period of the input signal, error in the extraction of theidentification signal occurs depending on the relationships between thereception start and stop timings of the input signal and the start andend timings of the period detection, and this error becomes more andmore noticeable as the detection cycle is prolonged. In the DB signalreceiving circuit 30, the control circuit 37 detects the signalcontinuance time based on the extraction timing of the identificationsignal, to determine a valid length as the selection signal. With themethod in which the period detection is performed over numerous periodsof the input signal, therefore, error in the detection of the signalcontinuance time expands because of large error in the extraction timingof the identification signal, giving rise to a problem that erroneousoperation occurs in relation to the output of number data.

Also, the input to the PB signal receiving circuit 30 very oftenincludes, besides the original PB signal, ambient noise or voice pickedup by the microphone of a telephone terminal as well as noise caused bycrosstalk of lines, etc., as mentioned above, and such noises must notbe detected as the identification signal. However, with the method inwhich the period detection is performed over numerous periods of theinput signal, the input signal is judged to be an identification signalinsofar as a predetermined number of periods of the input signal isdetected within a prescribed detection time, even if the input frequencygreatly varies during the detection cycle, so that erroneous detectionof the identification signal is liable to occur.

On the other hand, in the case of the method wherein the perioddetection is performed over a fewer periods of the input signal, if thefrequency range is broadened to admit noise at the time of frequencydetection, then the forbidden frequency deviation needs to be enlarged,as mentioned above. This, however, lowers the frequency detectionaccuracy and also often causes erroneous operation in relation to thedetermination of the presence/absence of input signal.

SUMMARY OF THE INVENTION

The present invention was created in view of the above circumstances,and an object thereof is to provide a PB signal receiving circuit whichhas enhanced frequency detection accuracy and yet is reduced inerroneous determination as to the valid length of PB signal and thus inerroneous operation.

To achieve the above object, there is provided a push-button signalreceiving circuit for receiving a push-button signal and identifying adialed number based on the received push-button signal. The push-buttonsignal receiving circuit comprises first and second frequency detectingportion provided respectively for low- and high-frequency identificationsignals included in the received push-button signal, for detectingfrequencies of input signals over one or more periods thereof to extractthe identification signals, third and fourth frequency detecting portionprovided respectively for the low- and high-frequency identificationsignals included in the received push-button signal, for detecting thefrequencies of the input signals over a number of periods greater thanthat of the first and second frequency detecting portion, to extract theidentification signals, valid signal determining portion for determiningwhether the received push-button signal is valid or not based on resultsof determination as to coincidence of continuance times of theidentification signals extracted by the first and second frequencydetecting portion, coincidence of the frequencies detected by the firstand third frequency detecting portion, and coincidence of thefrequencies detected by the second and fourth frequency detectingportion, and dialed number output portion for outputting the dialednumber based on the detected low and high frequencies if the push-buttonsignal is judged valid by the valid signal determining portion.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the principle of a PB signal receivingcircuit according to the present invention;

FIG. 2 is a diagram schematically showing an exemplary configuration ofthe PB signal receiving circuit according to the present invention;

FIG. 3 is a diagram showing an example of circuit configuration of afrequency detection circuit;

FIG. 4 is a chart showing output signals of various parts of thefrequency detection circuit, wherein FIG. 4(A) shows how a signal withan arbitrary frequency is detected, and FIG. 4(B) shows how a 697-Hzsignal is detected;

FIG. 5 is a chart showing signals appearing in various parts of the PBsignal receiving circuit;

FIG. 6 is a diagram schematically showing an exemplary configuration ofa conventional PB signal receiving circuit generally used;

FIG. 7 is a timing chart showing time-based changes of signals appearingin various parts of the PB signal receiving circuit;

FIG. 8 is a chart showing frequency detection characteristics of the PBsignal receiving circuit;

FIG. 9 is a chart illustrating variations in period caused by noiseduring period measurement;

FIG. 10 is a graph showing the relationship between the amplitude ratioof noise to signal and jitter;

FIG. 11 is a chart illustrating influence of jitter on the frequencydetection characteristics;

FIG. 12 is a chart showing examples of frequency detection over multipleperiods of an input signal, wherein (A) shows the input signal, (B)shows the frequency determination according to a first example ofdetection, (C) shows the output of an identification signal according tothe first example of detection, (D) shows the frequency determinationaccording to a second example of detection, and (E) shows the output ofthe identification signal according to the second example of detection;and

FIG. 13 is a chart showing examples of frequency detection over multipleperiods of the input signal in the case where a short break occurs,wherein (A) shows the input signal, (B) shows the frequencydetermination according to a first example of detection, (C) shows theoutput of the identification signal according to the first example ofdetection, (D) shows the frequency determination according to a secondexample of detection, and (E) shows the output of the identificationsignal according to the second example of detection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereinafter described withreference to the drawings.

FIG. 1 illustrates the principle of a push-button signal (hereinafterabbreviated as PB signal) receiving circuit according to the presentinvention.

As shown in FIG. 1, a PB signal receiving circuit according to thepresent invention comprises first and second frequency detecting portion2 and 3 provided respectively for low- and high-frequency identificationsignals included in a received PB signal, for detecting frequencies ofthe identification signals over one or more periods thereof, third andfourth frequency detecting portion 4 and 5 for detecting the frequenciesof the respective identification signals over a number of periodsgreater than that of the first and second frequency detecting portion,valid signal determining portion 6 for determining whether the receivedPB signal is valid or not, and dialed number output portion 7 foroutputting a dialed number specified by the PB signal, in accordancewith the result of determination by the valid signal determining portion6.

The first and second frequency detecting portion 2 and 3 performfrequency detection over a relatively small number of periods, forexample, one period, of the respective identification signals, toextract the low- and high-frequency identification signals,respectively. The third and fourth frequency detecting portion 4 and 5carry out frequency detection over a relatively large number of periodsof the respective identification signals, to extract the low- andhigh-frequency identification signals, respectively. Also, the first andsecond frequency detecting portion 2 and 3, for example, admit a widerrange of noise included in the input signal than the third and fourthfrequency detecting portion 4 and 5.

Each frequency detecting portion detects, for example, the period of theinput signal to detect its frequency. As a period detection method, amethod may be employed wherein, for example, a time required until thenumber of times the input signal level crosses a certain thresholdreaches a predetermined number is measured, and it is determined whetheror not the measured time falls within an allowable time range preset foreach frequency, to extract the identification signal with a desiredfrequency. In this case, the first and second frequency detectingportion 2 and 3, for example, have a wider allowable time range set perperiod of the respective identification signals than the third andfourth frequency detecting portion 4 and 5, thus setting differentallowable ranges for noise included in the input signal.

The valid signal determining portion 6 determines whether the receivedPB signal is valid or not based on the results of determination as tothe coincidence of continuance times of the identification signalsextracted by the first and second frequency detecting portion 2 and 3,the coincidence of frequencies detected by the first and third frequencydetecting portion 2 and 4, and the coincidence of frequencies detectedby the second and fourth frequency detecting portion 3 and 5.

Specifically, the continuance times of the identification signalsextracted by the first and second frequency detecting portion 2 and 3,for example, are measured. If the frequencies detected by the first andthird frequency detecting portion 2 and 4 coincide with each other andalso if the frequencies detected by the second and fourth frequencydetecting portion 3 and 5 coincide with each other when the continuancetimes reach a fixed time, it is judged that the PB signal being receivedis valid. Also, if any of the extracted identification signals does notcontinue for the fixed time but discontinues in the middle, theidentification signal is judged invalid. In the case where theextraction of the identification signal discontinues for a certain timeafter the signal is judged valid, it is concluded that the valid lengthof the PB signal has terminated.

When the PB signal is judged valid by the valid signal determiningportion 6, the dialed number output portion 7 identifies a specifieddialed number based on the detected low and high frequencies, andoutputs the dialed number. The output of the dialed number is started attiming at which the PB signal being received is judged valid by thevalid signal determining portion 6.

The first and second frequency detecting portion 2 and 3 detectfrequencies over a relatively small number of periods, and thus error inthe extraction start and stop timings of the identification signals issmall. Using the identification signals extracted by the first andsecond frequency detecting portion 2 and 3, which perform frequencydetection over a relatively small number of periods as mentioned above,the valid signal determining portion 6 measures the continuance time anddiscontinuance time of the signals to determine the validity of thesignals, whereby the timing for determining validness can be recognizedwith higher accuracy.

In cases were the frequency detection is performed over a relativelysmall number of periods n this manner, the frequency detection accuracylowers. Accordingly, using the third and fourth frequency detectingportion 4 and 5 for performing the frequency detection over a largernumber of periods, frequencies are detected in parallel, and only whenthe detected frequencies coincide, the PB signal is judged valid. Thisprevents erroneous operation from being caused either by error in thedetection of the frequencies of the identification signals or by errorin the recognition of the extraction start time, while allowinginclusion of a certain degree of noise.

An embodiment of the present invention will be now described. FIG. 2schematically illustrates an exemplary configuration of a PB signalreceiving circuit according to the present invention.

The PB signal receiving circuit 10 shown in FIG. 2 comprises a filter 11for removing a dial-tone signal from an input signal, filters 12 and 13for separating the input signal into low- and high-frequency signals,respectively, frequency detection circuits 14 and 15 for detectingfrequencies of the low- and high-frequency signals, respectively, over asmall number of periods thereof, frequency detection circuits 16 and 17for detecting the frequencies of the low- and high-frequency signals,respectively, over a large number of periods thereof, signaldetermination circuits 18 and 19 each for identifying a dialed numberbased on the frequencies detected by the frequency detection circuits14, 15 and 16, 17, a coincidence detection circuit 20 for determiningwhether or not the dialed numbers identified by the signal determinationcircuits 18 and 19 coincide with each other, a control circuit 21 fordetermining a valid length as a selection signal, and an output circuit22 for outputting number data having a valid length.

The filter 11 removes, from the input signal, the dial-tone signal whichis an audible signal with a frequency lower than the low-frequencysignals. The filters 12 and 13 selectively pass low- and high-frequencybands, respectively, of the signal output from the filter 11.

The frequency detection circuits 14 and 15 detect the frequencies of theoutput signals from the filters 12 and 13, respectively, over a smallnumber of periods, for example, one period, and extract low- andhigh-frequency identification signals, respectively. In parallel withthis, the frequency detection circuits 16 and 17 detect the frequenciesof the output signals from the filters 12 and 13, respectively, over alarger number of periods, and similarly extract the low- andhigh-frequency identification signals, respectively. The frequencydetection circuits 14 and 15 have frequency detection characteristicsthereof set so that a range for admitting noise may be wide as comparedwith the frequency detection circuits 16 and 17. Also, the frequencydetection circuits 16 and 17 do not perform their frequency detectionwhile no detection signal is input from the signal determination circuit18, described below.

When valid low and high frequencies are both detected by the frequencydetection circuits 14 and 15, respectively, the signal determinationcircuit 18 concludes that the identification signals have been detected,and outputs a detection signal to the control circuit 21 as well as tothe frequency detection circuits 16 and 17. Also, the signaldetermination circuit 18 determines a dialed number based on thecombination of the identification signals, and outputs number dataindicative of the result of determination to the coincidence detectioncircuit 20 and the output circuit 22 as 4-bit data D11, D12, D13 andD14, for example. Similarly, when valid low and high frequencies areboth detected by the frequency detection circuits 16 and 17,respectively, the signal determination circuit 19 outputs a detectionsignal indicative of the detect-on of identification signals to thecontrol circuit 21. Also, the signal determination circuit 19 outputsnumber data indicative of the result of determination to the coincidencedetection circuit 20 as 4-bit data D15, D16, D17 and D18, for example.

If the data D11 to D14 from the signal determination circuit 13 coinciderespectively with the data D15 to D18 from the signal determinationcircuit 19, the coincidence detection circuit 20 outputs a coincidencedetection signal, which indicates coincidence of the dialed numbersdetermined through the frequency detections over different detectioncycles, to the control circuit 21.

The control circuit 21, which is supplied with the detection signalsrelating to the respective frequency detections as well as thecoincidence detection signal, monitors the continuance time anddiscontinuance time of the detection signal from the signaldetermination circuit 18 to determine a valid length as theidentification signal, and outputs a validity signal EN indicative ofthe length. The output of the validity signal EN is started if the inputof the detection signal from the signal determination circuit 18 hascontinued for a fixed time and also if the control circuit is beingsupplied with the coincidence detection signal, and is stopped if theinput of the detection signal has discontinued for a second fixed time,as described later. The output circuit 22 outputs number data having alength based on the validity signal EN from the control circuit 21, as4-bit data D1, D2, D3 and D4, for example.

In the PB signal receiving circuit 10 described above, the frequencydetection circuits 14 and 15 perform frequency detection over a smallnumber of periods while admitting a wider range of noise, so that theextraction start and stop timings for the identification signal can bedetected with accuracy. Also, the frequency detection circuits 16 and 17carry out frequency detection over a large number of periods whileadmitting a narrower range of noise, so that the frequency detection canbe performed with accuracy.

FIG. 3 shows an exemplary circuit configuration of the frequencydetection circuit 14.

The frequency detection circuit 14 shown in FIG. 3 comprises an ADconversion section 141 including an AD converter circuit 141 a, a signalstart point detection section 142 including a latch circuit 142 a and anAND circuit 142 b, a signal period measurement section 143 including acounter circuit 143 a and a count holding circuit 43 b, and an outputsection 144 including comparator circuits 144 a, 144 b, 144 c and 144 d.

FIG. 4 shows output signals of various parts of the frequency detectioncircuit 14, wherein FIG. 4(A) shows how a signal with an arbitraryfrequency is detected, and FIG. 4(B) shows how a 697-Hz signal isdetected. In the following, operation of the frequency detection circuit14 will be described first with reference to FIGS. 3 and 4(A).

The AD converter circuit 141 a is supplied with an input signal S11 fromthe filter 12 and a reference signal SG and, in accordance with thesignal level of the input signal S11 relative to the reference signalSG, outputs a signal S12 which is a binary signal derived frog the inputsignal S11, as shown in FIG. 4(A). The signal start point detectionsection 142 latches, by portion of the latch circuit 142 a and the ANDcircuit 142 b, the input signal S12 synchronously with the rise of aclock signal CLK, and outputs a signal S13 which is a pulse wave with aduration corresponding to one period of the clock signal CLK, as shownin FIG. 4(A), thereby detecting a period start point of the input signalS11.

In the signal period measurement section 143, the counter circuit 143 akeeps counting up the clock signal CLK until it is supplied with thesignal S13. At the rise timing of the clock signal CLK following theinput of the signal S13, a count C11 is reset, as shown in FIG. 4 (A).Also, upon input of the signal S13, the count holding circuit 143 bholds the count C11 of the counter circuit 143 a at the rise timing ofthe succeeding pulse of the clock signal CLK, and outputs the held valueas a count C12. Thus, the count C12 counted at regular intervals isoutput from the signal period measurement section 143 for a time periodfrom the period start point to succeeding period start point of theinput signal S11.

In the output section 144, the comparator circuits 44 a to 144d, whichare associated respectively with low frequencies of 697 Hz, 770 Hz, 852Hz and 941 Hz, determine whether or not the input count C12 is betweenrespective upper- and lower-limit values for determining the respectivefrequencies, and output frequency determination signals S14, S15, S16and S17, respectively, to the signal determination circuit 18.

Table 1 below shows, by way of example, allowable frequencies, forbiddenfrequencies and criterion frequencies for the respective low- andhigh-frequency signals.

TABLE 1 Forbidden Allowable Allowable Forbidden Criterion CriterionCriterion Criterion frequency frequency frequency frequency frequencyfrequency count count (upper (lower (upper (lower (lower (upper (lower(upper Nominal limit) limit) limit) limit) limit) limit) limit) limit)value fo fpl fal fah fph fjl fjh nfjl nfjh 697 672.6 686.5 707.5 721.4679.6 714.4 1400 1472 770 743.1 758.5 781.6 797.0 750.8 789.3 1267 1332852 822.2 839.2 864.8 881.8 830.7 873.3 1145 1204 941 908.1 926.9 955.1973.9 917.5 964.5 1037 1090 1209 1166.7 1190.9 1227.1 1251.3 1178.81239.2 807 848 1336 1289.2 1316.0 1356.0 1382.8 1302.6 1369.4 730 7681477 1425.3 1454.8 1499.2 1528.7 1440.1 1513.9 661 694 1633 1575.81608.5 1657.5 1690.2 1592.2 1673.8 597 628

In Table 1, allowable frequencies fah and fal in each row respectivelyrepresent upper- and lower-limit values of frequency for allowingreception, set with respect to a corresponding nominal frequency fo.Also, forbidden frequencies fph and fpl respectively denote lower- andupper-limit values defining ranges in which reception is prohibited. Theforbidden frequencies fph and fpl are defined as +3.5% and −3.5%,respectively, of the nominal value fo, and the allowable frequencies fahand fal are usually set at +1.5% and −1.5%, respectively, of the nominalvalue fo. Criterion frequencies fjh and fjl respectively exemplifyupper- and lower-limit values for signal with a frequency to beidentified by the corresponding frequency detection circuit 14-17, andin the illustrated example, are set to +2.5% and −2.5%, respectively, ofthe nominal value fo. Criterion counts nfjh and nfjl exemplify countvalues corresponding respectively to the criterion frequencies fjh andfjl in the case where the clock frequency is 1 MHz.

Specifically, where the values shown in Table 1 are employed, thefrequency detection circuit 14 operates in the manner described below,for example. If the count C11 of the counter circuit 143 a is “1434”when the signal S13 is output, as shown in FIG. 4(B), data indicating“1434” is output as the count C12 from the count holding circuit 143 bat the rise timing of the succeeding pulse of the clock signal CLK.Since this value falls within the range between the criterion countsnfjh and nfjl for the frequency 697 Hz, the comparator circuit 144 aoutputs the signal S14. Consequently, the signal determination circuit18 recognizes that the 697-Hz identification signal has been extracted.

In the above example of frequency detection, period is measured over oneperiod of the input signal S11 for frequency detection. In cases whereperiod is to be measured over multiple periods, a counter is interposedbetween the signal start point detection section 142 and the signalperiod measurement section 143, for example, to count the number ofoutputs of the signal S13, and when the count has reached apredetermined number, a pulse signal with a duration corresponding toone period of the clock signal CLK is output from the counter. Also, ineach of the comparator circuits 144 a to 144 d are set criterion countsnfjh and nfjl corresponding to the number of periods to be measured.

In this case, therefore, the upper limit for the number of outputs ofthe signal S13 is set to a larger value in the frequency detectioncircuits 16 and 17 than in the frequency detection circuits 14 and 15.Also, to widen the noise admittance range, a criterion range per perioddetermined by the criterion counts nfjh and nfjl is set wider in thefrequency detection circuits 14 and 15 than in the frequency detectioncircuits 16 and 17. Consequently, even if the amount of jitter of asignal caused due to noise is relatively large, the frequency detectioncircuit 14, 15 identifies the signal as a signal with the predeterminedfrequency.

FIG. 5 shows signals appearing in various parts of the PB signalreceiving circuit 10. Operation of the PB signal receiving circuit 10will be now described with reference to FIGS. 2 and 5.

An input signal S21 including the dial-tone signal and low- andhigh-frequency signals is converted by the filter 11 to a signal S22from which the dial-tone signal has been removed. The signal S22 isfurther converted by the filters 12 and 13 to signals S23 and S24including only low and high frequencies, respectively.

The frequency detection circuits 14 and 15 perform period detection overa small number of periods of the input signals S23 and S24,respectively, and output signals S25 and S26 with low and highfrequencies, respectively. The high level of the signal S25, forexample, indicates that any one of the signals S14 Lo S17 is beingoutput from a corresponding one of the comparator circuits 144 a to 144d shown in FIG. 3. On detecting the input of both the signals S25 andS26 from the frequency detection circuits 14 and 15, the signaldetermination circuit 18 determines a dialed number based on thedetected low and high frequencies, and outputs number data indicative ofthe result of determination to the coincidence detection circuit 20 andthe output circuit 22 as 4-bit data D11, D12, D13 and D14. Also, thesignal determination circuit 18 outputs a signal S27, which is a logicalproduct of the input signals S25 and S26, to the frequency detectioncircuits 16 and 17 as well as the control circuit 21.

While the signal S27 from the signal determination circuit 18 is at theH level, the frequency detection circuits 16 and 17 perform perioddetection over numerous periods of the signals S23 and S24 input fromthe filters 12 and 13, respectively, and output signals S28 and S29 withlow and high frequencies, respectively. On detecting the input of boththe signals S28 and S29 from the frequency detection circuits 16 and 17,the signal determination circuit 19 determines a dialed number based onthe detected low and high frequencies, and outputs number dataindicative of the result of determination to the coincidence detectioncircuit 20 as 4-bit data D15, D16, D17 and D18. Also, the signaldetermination circuit 19 outputs a signal S30, which is a logicalproduct of the signals S28 and S29, to the control circuit 21.

The coincidence detection circuit 20 determines whether or not thenumber data determined by the signal determination circuit 18 coincideswith that determined by the signal determination circuit 19. To thisend, the coincidence detection circuit 20 derives an exclusive OR ofeach of the data D11 to D14 from the signal determination circuit 18 anda corresponding one of the data D15 to D18 from the signal determinationcircuit 19, and outputs a coincidence detection signal S31, which is aNOT signal of the logical sum of all the exclusive 0R's, to the controlcircuit 21.

The control circuit 21 monitors the signal S27 supplied from the signaldetermination circuit 18. Then, in accordance with the continuance timeand discontinuance time of the signal S27 and the coincidence detectionsignal S31 from the coincidence detection circuit 20, the controlcircuit outputs a validity signal EN indicative of a valid period of thereceived PB signal to the output circuit 22.

The signal S27 from the signal determination circuit 18 is generatedupon detection of the identification signals through the frequencydetection over a small number of periods, and therefore, the rise andfall timings of the signal S27 indicates, with relatively high accuracy,the reception start and stop timings of the PB signal. Accordingly, bymeasuring the continuance time and discontinuance time of the signalS27, the control circuit determines whether the received PB signal isvalid or not and also detects a minimum pause.

As shown in FIG. 5, on receiving the input signal S27 at timing T11, thecontrol circuit 21 starts to measure the continuance time of the signalS27. The signal S31 is obtained through the frequency detection overnumerous periods, and therefore, the input start timing thereof is laterthan that of the signal S27.

When the continuance time of the signal S27 reaches a preset continuancecriterion time Ton at timing T12, the control circuit looks up thecoincidence detection signal S31, and starts to output the validitysignal EN if the coincidence detection signal is then at the H level.Thus, the coincidence detection signal S31 is looked up when thecontinuance criterion time Ton has elapsed, and this permits thevalidity signal EN to be output only when the dialed number identifiedbased on the frequency detection by the frequency detection circuits 14and 15 coincides with the dialed number identified based on thefrequency detection by the frequency detection circuits 16 and 17.

As mentioned above, the frequency detection circuits 14 and 15 performfrequency detection over a smaller number of periods and have a widernoise admittance range, so that the frequency detection accuracy isrelatively low, possibly causing erroneous recognition of the dialednumber. On the other hand, the frequency detection circuits 16 and 17carry out frequency detection over a larger number of periods and have anarrower noise admittance range, so that the frequency detectionaccuracy is high. Accordingly, the validity signal EN is output onlywhen the coincidence detection signal S31 is being received, and onlywhile the higher-accuracy frequency detection is performed, the receivedPB signal is judged valid, whereby erroneous operation attributable toerror In the frequency detection over a small number of periods can beprevented.

When the input of the signal S27 discontinues at timing T13, the controlcircuit 21 starts to measure the discontinuance time. If the signal S27is again input thereafter at timing T14 before the discontinuance timereaches a preset discontinuance criterion time Toff, the control circuit21 judges that the discontinuance of the signal S27 from T13 to T14 isnot a minimum pause, and thus holds the output of the validity signal ENas it is during the discontinuance time.

Subsequently, when the input of the signal S27 again discontinues attiming T15, the control circuit 21 again starts to measure thediscontinuance time. At timing T16 after a lapse of the discontinuancecriterion time Toff, the control circuit judges that this discontinuanceis a minimum pause, and thus terminates the output of the validitysignal EN.

When the output of the signal S27 from the signal determination circuit18 discontinues, the frequency detection circuits 16 and 17 stop theirfrequency detection, and accordingly, the output of the signal S30 fromthe signal determination circuit 19 also discontinues at the same time.

The output circuit 22 derives a logical product of each of the data D11to D14 from the signal determination circuit 18 and the validity signalEN from the control circuit 21, and outputs the results as data D1 toD4. Consequently, the dialed number then detected is output at accuratereception start and stop timings of the PB signal in response to thevalidity signal EN, whereby the dialed number can be accuratelyrecognized.

As mentioned above, the PB signal receiving circuit 10 determines thevalidity of the input PB signal by using the signal S27 obtained throughthe frequency detection over a small number of periods, so that accuratereception timing of the PB signal can be detected. Also, at this time,it is determined whether or not the result of the frequency detectionperformed over a small number of periods coincides with the result ofthe frequency detection performed over a large number of periods, toenhance the frequency detection accuracy and thereby prevent error inthe determination of dialed number.

Accordingly, even in cases where noise caused by ambient sound picked upthe microphone of a telephone terminal, crosstalk of lines, etc. issuperimposed and as a result the waveform of the received signalincludes jitter, the influence of such jitter on the frequency detectioncan be lessened, thus improving the reception characteristics againstnoise. Also, since the forbidden frequency deviation can be reduced, itis possible to lower the rate of occurrence of erroneous operations suchas erroneous recognition of a continuous signal included in speech ormusic during communication, for example, as the PB signal.

As described above, in the PS signal receiving circuit according to thepresent invention, the validity of the received PB signal is determinedin accordance with the results of determination as to the coincidence ofthe continuance times of the identification signals extracted by thefirst and second frequency detecting portion for performing thefrequency detection over a small number of periods, and the coincidenceof the frequencies detected by the first and second frequency detectingportion with those detected by the third and fourth frequency detectingportion over a large number of periods. It is therefore possible toprevent erroneous operation attributable to either error in thefrequency dejection of the identification signals or error in therecognition of the extraction start timing while allowing inclusion of acertain degree of noise.

The foregoing is considered as illustrative only of the principles ofthe present invention. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and applications shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be regarded as falling within the scope of the invention n theappended claims and their equivalents.

1. A push-button signal receiving circuit for receiving a push-buttonsignal and identifying a dialed number based on the received push-buttonsignal, comprising: first and second frequency detecting portionprovided respectively for low- and high-frequency identification signalsincluded in the received push-button signal, for detecting frequenciesof input signals over one or more periods thereof to extract theidentification signals; third and fourth frequency detecting portionprovided respectively for the low- and high-frequency identificationsignals included in the received push-button signal, for detecting thefrequencies of the input signals over a number of periods greater thanthat of said first and second frequency detecting portion, to extractthe identification signals; valid signal determining portion fordetermining whether the received push-button signal is valid or notbased on results of determination as to coincidence of continuance timesof the identification signals extracted by said first and secondfrequency detecting portion, coincidence of the frequencies detected bysaid first and third frequency detecting portion, and coincidence of thefrequencies detected by said second and fourth frequency detectingportion; and dialed number output portion for outputting the dialednumber based on the detected low and high frequencies if the push-buttonsignal is judged valid by said valid signal determining portion, whereinsaid first, second, third and fourth frequency detecting portion eachmeasure a time required until the number of times the signal level ofthe input identification signal crosses a threshold reaches apredetermined number, determine whether or not the measured time fallswithin an allowable time range, and detect the frequency of theidentification signal, and wherein the allowable time range set in saidfirst and second frequency detecting portion per period of theidentification signal is wider than that set in said third and fourthfrequency detecting portion.
 2. The push-button signal receiving circuitaccording to claim 1, wherein said valid signal determining portionjudges the received push-button signal to be valid if the identificationsignals extracted by said first and second frequency detecting portioncontinue for a fixed time and if the frequencies detected by said firstand third frequency detecting portion coincide and if the frequenciesdetected by said second and fourth frequency detecting portion coincide.3. The push-button signal receiving circuit according to claim 1,wherein said valid signal determining portion judges a valid length ofthe push-button signal to be terminated if the identification signalsextracted by said first and second frequency detecting portion havediscontinued for a fixed time.
 4. The push-button signal receivingcircuit according to claim 1, wherein said third and fourth frequencydetecting portion start the frequency detection when the identificationsignals are extracted by said first and second frequency detectingportion.
 5. The push-button signal receiving circuit according to claim1, wherein said third and fourth frequency detecting portion terminatethe frequency detection when the identification signals extracted bysaid first and second frequency detecting portion has discontinued.
 6. Apush-button signal detection method for receiving a push-button signaland identifying a dialed number based on the received push-buttonsignal, comprising: performing first and second frequency detectionprocesses each with respect to low- and high-frequency identificationsignals included in the received push-button signal, said firstfrequency detection process being a process for detecting frequencies ofinput signals over one or more periods thereof to extract theidentification signals, said second frequency detection process being aprocess for detecting the frequencies of the input signals over a numberof periods greater than that of said first frequency detection process,to extract the identification signals; judging that the receivedpush-button signal is valid if the frequencies detected in said firstand second frequency detection processes coincide with each other whenthe identification signals extracted in said first frequency detectionprocess have continued for a fixed time; and outputting the dialednumber based on the detected low and high frequencies if the receivedpush-button signal is judged valid, wherein said first, second, thirdand fourth frequency detecting portion each measure a time requireduntil the number of times the signal level of the input identificationsignal crosses a threshold reaches a predetermined number, determinewhether or not the measured time falls within an allowable time range,and detect the frequency of the identification signal, and wherein theallowable time range set in said first and second frequency detectingportion per period of the identification signal is wider than that setin said third and fourth frequency detecting portion.